As semiconductor devices become more highly integrated, the size of cell regions in the semiconductor devices are being reduced. Accordingly, forming a capacitor having a capacitance sufficient for stable operation of a cell may be difficult. The capacitance “C” of a capacitor is directly proportional to the dielectric constant “∈” (∈=∈o∈r) of a dielectric layer and the area “A” of the electrodes, and is inversely proportional to the distance “d” between the electrodes, as shown by the following equation.C=∈A/d 
For capacitors formed using a conventional dielectric material such as silicon oxide or silicon nitride, methods of forming cylindrical-shaped or fin-type lower electrodes have been developed to increase the effective area of the capacitor, thereby increasing the capacitance. However, forming an electrode having such a complicated shape may be difficult.
In order to address the above-mentioned problems, methods of forming a dielectric layer using a material having a relatively high dielectric constant have been investigated. For example, metal oxides having a relatively high dielectric constant, such as aluminum oxide (Al2O3), tantalum oxide (Ta2O5), niobium oxide (Nb2O5), zirconium dioxide (ZrO2), titanium dioxide (TiO2), and the like, have been developed for use as the dielectric material between the electrodes of a capacitor for a DRAM device in the gigabyte class. The dielectric constants of the above-mentioned materials are in a range of about 10 to about 114, which is about 2.5 to 30 times as large as the dielectric constant (3.9) of silicon oxide (SiO2).
Generally, a thin film such as a dielectric layer may be deposited by a process such as a chemical vapor deposition (CVD) process, a low-pressure CVD (LPCVD) process, a plasma-enhanced CVD (PECVD) process or a sputtering process. When a process for forming a thin film using CVD is performed, the thin film may be formed at a relatively high temperature and thus a device including the thin film may be disadvantageously affected by the elevated temperature. Additionally, a thin film formed by a CVD process may have an irregular thickness and may have poor step coverage characteristics.
In contrast, atomic layer deposition (ALD) processes are generally performed at a temperature lower than that of CVD processes, and thin films formed by ALD processes may have suitable composition characteristics, relatively uniform thickness and desirable step coverage characteristics. Thus, ALD processes may be desirable for forming gate oxide layers and/or dielectric layers of a capacitor having a high dielectric constant.
A typical ALD process may include the following steps. First, a first source gas including a first element may be introduced into a chamber so that the first element gas may be chemically adsorbed (chemisorbed) onto a substrate. The remaining portion of the first source gas in the chamber that has not been chemisorbed onto the surface may then be purged by an inert gas. Next, a second source gas including a second element may be introduced into the chamber so that the first element chemisorbed onto the substrate and the second source gas may react with each other to form a layer including the first and second elements. Then, a unreacted portion of the second source gas in the chamber and the by-products generated by the reaction are purged by an inert gas. The above processes may be repeatedly performed until a thin film with the desired thickness is formed.
Precursors used in ALD processes should desirably have the following characteristics. First, the precursor should have a relatively high saturated vapor pressure and should be relatively chemically and thermally stable. Second, ligands bonded to a metal should dissociate relatively quickly and completely. Third, organic materials from the precursor should be absent or minimal in the final thin film. Fourth, the precursor should be relatively nontoxic and in a liquid state at room temperature. Fifth, the precursor should deposited at a relatively high deposition rate. Sixth, the precursor should have a relatively high purity and a relatively low cost.
Conventional precursors, such as alkyl metals, metal alkoxides, metal halides, β-diketonates, may not have all of the above-mentioned characteristics. For example, an alkyl metal precursor such as Pb(C2H5)4 has been shown to be toxic and explosive. Additionally, some metal alkoxide precursors may be sensitive to moisture, and thus a metal included in the metal alkoxide may easily combine with hydrogen or a hydroxyl group so that undesirable impurities such as metal hydroxide may be included in the metal oxide layer. Furthermore, β-diketonate precursors, which may be relatively expensive, may have a relatively low saturated vapor pressure and may be solid at room temperature, which may result in difficulties in processing. Beta-diketonate precursors, such as a hexafluoropentanedionate precursor and a heptafluorodimethyloctanedionate precursor, that have improved volatility have also been studied. However, ligands included in such precursors may not readily dissociate from the metal in the precursors because the reactivity of the precursors is relatively low. Additionally, the deposition rate of the precursors is low because the precursors have a relatively high molecular weight.
Tetrakis ethylmethylamino hafnium (TEMAH, Hf[N(CH3)(C2H5)]4) and tetrakis ethylmethylamino zirconium (TEMAZ, Zr[N(CH3)(C2H5)]4), which have also been studied, may be relatively thermally unstable so that mass production of such precursors may be difficult.